Assembly - Function computes the value of a number raised to a power

assembly_function.s

Assembly_Function

#PURPOSE:  Program to illustrate hoe functions work. Computes 2^3 + 5^5
# Everything in the main program is stored in registers, so the data section doesn't have anything

.section .data

.section .text

.global _start
_start:
pushl $3		#push second argument
pushl $2		#push first argument
call power		#call the function
addl  $8, %esp		#move the stack pointer back

pushl %eax		#save the first answer before calling the next function

pushl  $2		#push second argument
pushl $5		#push first argument
call power		#call function
addl $8, %esp		#move the stack pointer back

popl  %ebx		#The second answer is already in %eax. We saved the first answer onto the stack
			#So now we can just pop it out onto %ebx

addl  %eax, %ebx	#add them together and the result is in %ebx

movl  $1, %eax	#exit (%ebx is returned)
int  $ox80

#PURPOSE:  This function is used to compute the value of a number raised to a power.

#INPUT:  The first argument - base number, Second argument = the power to raise it to

#OUTPUT: Will give the result as a return value
#NOTES:  The power must be 1 or greater

#VARIABLES:  %ebx = holds the base number,  %ecx = holds the power
#  -4(%ebp) = holds the current result
#  %eax is used for temporary storage









#Start function
.type power, @function
power:
push1  %ebp 			#save old base pointer
movl  %esp, %ebp		#make stack pointer the base pointer
subl  $4, %esp			#get room for our local storage

movl  8(%ebp), %ebx 	#put first argument in %ebx
movl  12(%ebp), %ecx	#put second argumet in %ecx

movl  %ebx, -4(%ebp) 	#store current result

power_loop_start:
cmpl	$1, %ecx		#if the power is 1, we are done
je  end_power
movl   -4(%ebp), %eax	#move the current result into %eax
imull  %ebx, %eax		#multiply the crrent result by the base number
movl	%eax, -4(%ebp)	#store the current result

decl  %ecx			#decrease the power
jmp  power_loop_start	#run for the next power

end_power:
movl  -4(%ebp), %eax	#return value goes in %eax
movl  %ebp, %esp		#restore the stack pointer
popl  %ebp			#restore the base pointer
ret